Intumescent ablative composition

ABSTRACT

An ablative composition and methods of forming ablative structures are provided that improve char during ablation, prevent combustion during ablation, and which reduce moisture absorption of low temperature ablative (LTA) materials. The ablative composition comprises an intumescent material such as ammonium polyphosphate (APP) that is disposed within an LTA material at the outer surface of the ablative composition. The intumescent material may also be added in increasing amounts throughout the LTA material such that a gradient of intumescent material is formed near the outer surface of the ablative composition for the required amount of thermal protection. Both the LTA material and the intumescent material are applied to a substrate, or an aerospace vehicle structure, preferably in layers using methods such as spray forming or hand troweling.

FIELD OF THE INVENTION

The present invention relates generally to low temperature ablativecompositions and more particularly to compositions comprising quantitiesof intumescent materials in addition to methods for mixing thecompositions and forming ablative structures.

BACKGROUND OF THE INVENTION

Ablative materials have been used in a number of applications to protectand insulate objects that are subjected to extreme thermal conditions.More specifically, extreme thermal conditions in aerospace vehicles havebeen managed using a variety of techniques including insulation andradiant cooling, active cooling, conduction and convective cooling, andby phase change or ablative materials. Generally, ablative materials areapplied to the affected aerosurfaces and/or substructure to absorb theradiant and convective heat and to insulate the vehicle from the extremethermal environment.

Aerospace launch vehicles having solid rocket boosters generate highconvective and radiant heat near the base region of main engines. Toprevent damage from the high heat, structure near the engines istypically protected with a layer of low temperature ablative (LTA)material. The LTA material generally insulates the structure byabsorbing the heat through an ablation process, wherein the LTA materialforms a char and thereafter burns for a period of time. During exposureto extreme heating and subsequent ablation, the LTA material maydecompose and recede across its surface. The recession is generally dueto phase change processes such as melting, sublimation, or chemicalreactions including oxidation and combustion. Similarly, thedecomposition is due to processes such as pyrolysis, phase changes, orchemical reactions.

The performance of LTA materials is often characterized by “q*” or “heatof ablation,” which is defined as:

q*=qdot/mdot;

where:

qdot=q_(hw)−q_(rad); (net heat flux)

q_(hw)=convective hot wall flux;

q_(rad)=net radiative heat flux; and

mdot=rate of mass loss.

In order to adequately protect structure and systems from extremethermal conditions, LTA materials must have a high heat of ablation inaddition to low thermal conduction. Furthermore, LTA materials inaerospace applications typically have a low density in order to minimizeweight, and are further able to withstand a variety of flight loads,such as aerodynamic shear forces, in addition to extreme heating.

When LTA materials are exposed to high heat flux and oxygen from theatmosphere, the LTA materials quickly char and begin burning. Onceignited, the LTA materials may continue to burn even after the heatsource subsides. Accordingly, effective LTA materials typically form astrong char during the ablation process, which is sufficient to preventseparation of at least a portion of the LTA material from the structuredue to aerodynamic forces, thermal shock, and vibrations.

Generally, the char provides increased thermal protection because lessLTA material is removed during the ablation process. The char is alsoporous, lightweight, and has low thermal conductivity to further improvethermal protection. Additionally, radiant heat loss is increased sincethe char has higher emissivity and can withstand higher temperatures,and the higher temperatures further reduce convective heat gain.

Unfortunately, a critical failure mode of LTA materials is the formationof a weakened char. As the material forms a char and burns during theablation process, cracks may form in the surface of the LTA materials.The cracks typically increase in size over time and eventually cause theLTA material to fracture and erode away due to aerodynamic forces.Therefore, effective LTA materials must be capable of forming a strongchar.

LTA materials are also susceptible to moisture absorption due to theirporosity and lightweight. Moisture absorption increases the weight ofthe LTA material and further contributes to weakened char during theablation process. Accordingly, a thin layer of sealant or paint, such asCorlar®, is applied over the top of the LTA materials, as a coating, toreduce moisture absorption. Unfortunately, the application of a sealantor paint increases the weight of the ablative composition, and furtherincreases manufacturing cycle time and overall costs.

In addition to LTA materials, intumescent materials have also been usedin high heat applications. Intumescent materials, generally defined asmaterials that swell when heated, have been used extensively as thermalbarriers in the chemical and oil industries for fire protection.Unfortunately, intumescent materials have a high density and have beenundesirable for use in weight sensitive applications such as inaerospace vehicles. Furthermore, the char that is produced byintumescent materials after being subjected to flames cannot withstandhigh aerodynamic shear forces.

Accordingly, there remains a need in the art for a lightweight ablativecomposition and methods of forming ablative structures that reduces theamount of ablation, strengthens the char, and protects the LTA againstmoisture absorption while improving manufacturability and reducingoverall costs.

SUMMARY OF THE INVENTION

In one preferred form, the present invention provides an ablativecomposition that comprises a quantity of fire retarding intumescentmaterial disposed within a low temperature ablative (LTA) material. Toform an ablative structure, a quantity of LTA material is first appliedto a substrate, such as an aerospace vehicle structure. An intumescentmaterial is then mixed with a further quantity of LTA material, and themixture is applied to the substrate, over the top of the first quantityof LTA material. Accordingly, intumescent material is disposed withinthe LTA material at the outer surface of the ablative structure, orablative composition, to provide the requisite amount of thermalprotection.

Preferably, the intumescent material is mixed with the LTA materialduring a spray forming process, wherein the LTA material is firstdeposited onto a substrate in streams during multiple passes of severalspray heads to form an ablative structure. The intumescent material isthen added to the LTA material during the final pass of the spray headsat the outer surface of the ablative composition.

Alternately, other known methods may be employed to apply the ablativecomposition to the substrate, such as manual troweling or pre-formingfollowed by a secondary bonding operation to the substrate. In addition,the ablative composition is cured onto the substrate, preferably at roomtemperatures, for a period of time that depends on the materials usedand the amount of thermal protection required.

In another preferred form, the intumescent material is added to the LTAin increasing amounts towards the outer surface of the ablativecomposition, thereby forming a gradient of intumescent material. Anincreased amount of intumescent material is added to the LTA in eachsuccessive layer as layers of material are applied to the substrateusing, for example, spray forming or manual troweling methods. As aresult, the amount of intumescent material gradually increases towardsthe outer surface of the ablative composition for the required amount ofthermal protection.

Under extreme thermal conditions, the intumescent material causes theLTA material to swell, and as a result, the LTA expands outward to blockradiant heat and further expands inward to back-fill minor cracks. Theswelling further prevents external heating and ambient oxygen fromreaching the structure beneath the ablative composition. Advantageously,a stronger char is formed and the structure is adequately protected fromthe high heat.

Preferably, the LTA material is cork-based and further comprises epoxy.Additionally, the intumescent material is preferably ammoniumpolyphosphate (APP). The APP is added in a percentage betweenapproximately 10% and 50% to the LTA, and the thickness of each layerapplied to the substrate is between approximately 0.05 inches and 0.75inches. Furthermore, the LTA and intumescent materials are cured at roomtemperature for approximately 10 minutes to approximately 24 hours, andpreferably between approximately 1 and 4 hours. The amount of LTA andintumescent materials, and further the total thickness of the ablativecomposition, depends on the desired amount of thermal protection asdescribed in greater detail below.

By adding the intumescent material to the LTA during the formingprocess, the added step of applying a coating of sealant or paint toprevent moisture absorption is eliminated. The ablative composition ofthe present invention further reduces the amount of ablation,strengthens the char on the surface of the ablative structure, andprotects the LTA against moisture absorption.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a side sectional view of a layer of intumescent materialdisposed within a low temperature ablative material in accordance withthe present invention; and

FIG. 2 is a side sectional view of intumescent material disposed withina low temperature ablative material to form a gradient of intumescentmaterial near the outer surface of the ablative composition inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

Referring to the drawings, the ablative composition of the presentinvention is illustrated and generally indicated by reference numeral 10in FIG. 1. As shown, ablative composition 10 is applied to a substrate12, which may be the structure of an aerospace vehicle that is subjectedto extreme thermal conditions. The ablative composition 10 generallycomprises an intumescent material 14 disposed within a low temperatureablative (LTA) material 16.

The application of the ablative composition 10 to an aerospace vehicleshould not be construed as limiting the invention; rather theapplication to aerospace is merely illustrative of one structure and oneoperating environment in which the present invention has particularutility. The ablative composition of the present invention can furtherbe employed with a wide variety of objects that must withstand highthermal loads for an extended duration.

To form an ablative structure, a first quantity of LTA material 16 isfirst applied to the substrate 12. The intumescent material 14 is thenmixed with a further quantity of LTA material 16, and the mixture isthen applied to the substrate 12 over the top of the first quantity ofLTA material 16. Accordingly, the intumescent material 14 is disposedwithin the LTA material 16 at the outer surface 18 of the ablativecomposition 10 for the requisite amount of thermal protection.

Preferably, the LTA material 16 is applied to the substrate 12 in layersusing spray forming methods. However, LTA material 16 may also beapplied to substrate 12 using other known methods in the art such asmanual troweling or pre-forming followed by adhesive bonding to thesubstrate 12. After a sufficient amount of LTA material 16 is applied tothe substrate 12, the intumescent material 14 is then added to LTAmaterial 16 and applied in a final layer. The final layer may be appliedduring the final pass of a spray head, or multiple spray heads, suchthat the intumescent material 14 is disposed within the LTA material 16at the outer surface 18 of ablative composition 10.

After the LTA material 16 and the intumescent material 14 have beenapplied to the substrate 12, the ablative composition 10 is cured for apredetermined amount of time. Preferably, the ablative composition iscured at room temperature for approximately 10 minutes to approximately24 hours, and more specifically between approximately 1 and 4 hours. Itshall be appreciated by those skilled in the art that the curetemperature and time are a function of the type of materials used forthe LTA material 16 and the intumescent material 14.

Preferably, the intumescent material 14 is ammonium polyphosphate (APP)and the LTA material 16 is a cork epoxy such as MCC-1. The percentage ofintumescent material 14 that is added to the LTA material 16 is betweenapproximately 10 percent and 50 percent. Preferably, the percentage ofintumescent material 14 is between approximately 25 percent and 30percent. Additionally, the thickness per pass of ablative composition 10is between approximately 0.05 inches and 0.75 inches. Preferably, thethickness per pass of ablative composition 10 is approximately 0.25inches.

The total amount of ablative composition 10 that is applied to thesubstrate 12 depends on the amount of thermal protection required andthe particular LTA 16 material applied. One preferred embodiment of thepresent invention comprises an MCC-1 LTA material and an APP intumescentmaterial, wherein the ablative composition 10 has a total thickness ofapproximately 0.75 inches. The ablative composition 10 was designed andtested to protect a graphite epoxy substrate subjected to plumes of theDelta IV Medium-Plus (M+) solid rocket boosters. Generally, the requiredamount of LTA material 16 and intumescent material 14 are determined byexperimental testing in a controlled environment.

Preliminary testing of the ablative composition 10 according to thepresent invention resulted in a reduction in the amount of ablation byapproximately 30 to 50 percent. The test conditions simulated a Delta IVM+ vehicle trajectory and corresponding thermal environment. The amountof intumescent material 14 disposed within the LTA material 16 wasformulated to block the simulated plume for approximately 10 seconds ina high heat flux, but low shear environment. After approximately 10seconds, the vehicle is at a sufficient altitude such that the reducedamount of oxygen will suppress the flame. Further, after approximately20 seconds, the ambient oxygen does not support combustion. Accordingly,further preliminary testing has shown that preventing the ablativecomposition 10 from igniting during approximately the first 5 secondsresults in lower heat flux during flight and a reduced thickness of LTAmaterial.

In another preferred form of the present invention, intumescent material14 is added to LTA material 16 in increasing amounts with each pass orlayer, such that a gradient 20 of intumescent material 14 is formed atthe outer surface 18 of ablative composition 10 as shown in FIG. 2. Theablative composition 10 similarly undergoes a curing process aspreviously described to form the completed ablative structure. Dependingon the amount of thermal protection required, therefore, the amount ofintumescent material 14 can be tailored for an adequate amount ofthermal protection.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

What is claimed is:
 1. A method of forming an ablative structure, themethod comprising the steps of: (a) applying to a substrate a firstquantity of a low temperature ablative material that is absent anintumescent material; (b) mixing an intumescent material with a secondquantity of the low temperature ablative material; and (c) applying theintumescent material mixed with the second quantity of the lowtemperature ablative material over the first quantity of the lowtemperature ablative material.
 2. The method of claim 1, wherein the lowtemperature ablative material and the intumescent material are appliedto the substrate using spray forming.
 3. The method of claim 2, whereinthe intumescent material mixed with the quantities of low temperatureablative material is between approximately 0.05 and 0.75 inches thick.4. The method of claim 3, wherein the intumescent material mixed withthe quantities of low temperature ablative material is approximately0.25 inches thick.
 5. The method of claim 1, wherein the intumescentmaterial is mixed between approximately 10 and 50 percent by weight withthe low temperature ablative material.
 6. The method of claim 5, whereinthe intumescent material is mixed between approximately 25 and 40percent by weight with the low temperature ablative material.
 7. Themethod of claim 1 further comprising the step of curing the lowtemperature ablative material and the intumescent material onto thesubstrate.
 8. The method of claim 7, wherein the low temperatureablative material and the intumescent material are cured at roomtemperature.
 9. The method of claim 8, wherein the low temperatureablative material and the intumescent material are cured onto thesubstrate between approximately 10 minutes and 24 hours.
 10. The methodof claim 9, wherein the low temperature ablative material and theintumescent material are cured onto the substrate between approximatelyone and 4 hours.
 11. The method of claim 1, wherein the intumescentmaterial is ammonium polyphosphate.
 12. The method of claim 1, whereinthe low temperature ablative material is cork-based.
 13. A method offorming an ablative structure, the method comprising the steps of: (a)applying a first quantity of low temperature ablative material to asubstrate; (b) mixing a first quantity of intumescent material with asecond quantity of low temperature ablative material; (b) applying thefirst quantity of intumescent material mixed with the second quantity oflow temperature ablative to the substrate on top of the first quantityof low temperature ablative material; (c) mixing further quantities ofintumescent material with further quantities of low temperature ablativematerial; and (d) applying the further quantities of intumescentmaterial mixed with the further quantities of low temperature ablativematerial to the substrate on top of the first quantity of intumescentmaterial mixed with the second quantity of low temperature ablativematerial, wherein the further quantities of low temperature ablativematerial are successively less than the second quantity of lowtemperature ablative material, such that the intumescent material isapplied in an increasing amounts, thereby forming a gradient ofintumescent material.
 14. The method of claim 13, wherein the quantitiesof low temperature ablative material and the quantities of intumescentmaterial are applied to the substrate using spray forming.
 15. Themethod of claim 14, wherein the quantities of intumescent material mixedwith the quantities of low temperature ablative material is betweenapproximately 0.05 and 0.75 inches thick.
 16. The method of claim 15,wherein the quantities of intumescent material mixed with the quantitiesof low temperature ablative material is approximately 0.25 inches thick.17. The method of claim 13, wherein the intumescent material is mixedbetween approximately 10 and 50 percent by weight with the lowtemperature ablative material.
 18. The method of claim 17, wherein theintumescent material is mixed between approximately 25 and 40 percent byweight with the low temperature ablative material.
 19. The method ofclaim 13 further comprising the step of curing the low temperatureablative material and the intumescent material onto the substrate. 20.The method of claim 19, wherein the low temperature ablative materialand the intumescent material are cured at room temperature.
 21. Themethod of claim 20, wherein the low temperature ablative material andthe intumescent material are cured onto the substrate betweenapproximately 10 minutes and 24 hours.
 22. The method of claim 21,wherein the low temperature ablative material and the intumescentmaterial are cured onto the substrate between approximately one and 4hours.
 23. The method of claim 13, wherein the intumescent material isammonium polyphosphate.
 24. The method of claim 13, wherein the lowtemperature ablative material is cork-based.
 25. A method of forming anablative structure, the method comprising the steps of: (a) mixing aquantity of intumescent material with a low temperature ablativematerial, wherein the intumescent material is disposed within the lowtemperature ablative material in increasing amounts towards an outersurface of the ablative structure, thereby forming a gradient ofintumescent material; and (b) applying the quantity of intumescentmaterial mixed with the low temperature ablative material to asubstrate.